Lead free, tin-bismuth solder alloys

ABSTRACT

The lead free alloy is a low solidus temperature, multi-component solder alloy containing at least about 50 weight percent Bi, up to about 50 weight percent Sn (basis total Sn and Bi), and an effective amount of a physical and mechanical property enhancing third component. The third component can be Cu, In, Ag, and combinations of Cu and Ag.

FIELD OF THE INVENTION

The invention relates to lead free solders and especially to lead free,tin-bismuth solder alloys. These alloys are low toxicity solder alloysthat are particularly useful in microelectronic applications. The solderalloys of the invention are particularly useful in joining integratedcircuit chips to chip carriers and substrates, as printed circuitboards, joining chip carriers to substrates, and joining circuitizationlands and pads in multilayer printed circuit boards.

BACKGROUND OF THE INVENTION

Soldering is a low temperature, generally reversible, metallurgicaljoining process. Low temperature and reversibility are especiallyimportant in electronics applications because of the materials involvedand the necessity for reworking and making engineering changes.

Solder joining is a wetting process followed by a chemical reaction.Molten solder wets selectively. The selective wettability of soldersallow molten solder to be confined to desired sites. This is especiallyimportant in flip chip bonding, and in working with solder masks.

The soldering process can be accomplished as quickly as the wettingtakes place, for example, on the order of a few seconds. This makessoldering particularly desirable for automated, high speed, highthroughput processes.

Wettability is also a function of the materials to be joined, with Cu,Ni, Au, and Pd, as well as alloys rich in one or more of these metals,being particularly amenable to soldering.

The chemical reaction following wetting is between the molten solder andthe joining metallurgy to form an intermetallic phase region at theinterface. The intermetallic phases formed by solders in electronicpackaging are stoichiometric compounds, typically binary compounds, andtypically containing Sn if Sn is present in the solder alloy. When thebase, pad, or land is Cu, and the solder alloy is rich is Sn, theintermetallic formed during soldering is Cu--Sn. Exemplary Cu--Snbinaries include Cu₃ Sn and Cu₆ Sn₅.

Solder alloys are characterized by the melting temperature being astrong function of composition. While a pure metal is characterized by asingle, invariant, melting temperature, the freezing and melting pointsof alloys are complex. The freezing point of an alloy is determined bythe liquidus line. Above the liquidus line only a liquid phase or phasescan exist. The melting point of an alloy is determined by the solidusline. Below the solidus line only a solid phase or phases can exist. Inthe region between these two lines, i.e., between the liquidus line andthe solidus line, solid and liquid phases can co-exist.

The preferred soldering alloys are eutectics, that is, they arecharacterized by a eutectic point. The eutectic point is where theliquidus and solids lines meet. A concentration change in eitherdirection from the eutectic results in an increase in the liquidustemperature.

The composition, and the quench rate, also determine the microstructureand the resulting mechanical properties of the solder joint. Thus, it isnecessary to both carefully choose the solder composition and tocarefully control the thermal exposure of the soldered joint.

A solder composition used in electronics fabrication must be wettable asa solder alloy, and have at least one component capable of forming anelectrically conductive, thermally stable, non-brittle, plasticintermetallic with the pad or land metallurgy. For this reason, the mostcommon solder alloys are lead based alloys, as Sn--Pb alloys.

Heretofore, Pb/Sn solders have been utilized for electronicapplications. There have been many historical reasons for the widespread use of Pb/Sn alloys. These historical reasons include the lowsolidus temperature of Pb/Sn solder alloys, the workability of Pb/Snalloys and of the resulting Cu/Sn intermetallics (formed at thesolder/Cu contact interface) over a wide temperature range, the adhesionof Cu/Sn intermetallics obtained from Pb/Sn alloys to Cu lands and pads,and the ready availability of process equipment and low cost adjuncts,as resins, fluxes, and solder masks, for Pb/Sn alloys.

The relatively low temperatures required for processing Pb/Sn solderalloys are particularly important when polymeric dielectrics are used inthe fabrication of electronic packages. These polymers can degrade inhigh temperature assembly operations. Solder alloys which melt atrelatively low temperatures can accommodate these polymeric substrates.

Additionally, semiconductor chips are subject to thermal diffusion andstructural transformations at elevated temperatures. Low melting soldersavoid these problems.

Especially important is the "softness" or plasticity of lead basedsolders. This softness or plasticity allows the solder to accommodatethe mismatch in coefficients of thermal expansion between the bondedstructures, for example the mismatch in coefficient of thermal ofthermal expansion between a ceramic dielectric and a polymericdielectric, or between a semiconductor chip and a ceramic or polymericchip carrier or substrate.

However, lead is a toxic, heavy metal with a relatively high vaporpressure. Its use is disfavored, and a need exists for a replacement.

Exemplary lead-free alloys are described, for example in EP 0-354-570-A2to A. J. G. Strandjord, R. L. Yates, and D. J. Perettie for OPTICALINFORMATION STORAGE MEDIUM which describes an optical data storage mediacontaining an alloy of 40-85% Sn, 10-50% Bi, 1-15% Cu.

Lead free solder alloys are disclosed in, for example U.S. Pat. No.4,929,423 to K. L. Tucker and U. Ma for LOW TOXICITY ALLOY COMPOSITIONSFOR JOINING AND SEALING which describes a lead free solder alloy forjoining and sealing, disclosed to be useful as a plumbing alloy. Thedisclosed alloy contains 0.08-20% Bi, Cu, Ag, P, Rare Earth, and balance(ca. 80%) Sn

Seraphim, Lasky & Li, PRINCIPLES OF ELECTRONIC PACKAGING, shows, atTable 19.1, a lead free alloy containing 58% Bi and 42% Sn.

However, the above alloys fail to provide a lead free solder that flowsat low enough temperatures to avoid damage to electronic materials, wetsthe bonding metallurgy typically used in electronics fabrication, e.g.,Cu, Au, Ag, and Pd, especially while avoiding wetting organic materialsas substrates and solder masks.

OBJECTS OF THE INVENTION

It is a primary object of this invention to provide a lead free solder.

It is a further object of this invention to provide a lead free solderthat wets and forms a chemically and thermally stable intermediate thebonding metallurgy typically used in electronics fabrication, e.g., Cu,Au, Ag, and Pd, especially while avoiding wetting organic materials assubstrates and solder masks.

It is a further object of this invention to provide a lead free solderhaving that flows at low enough temperatures to avoid damage toelectronic materials.

SUMMARY OF THE INVENTION

These and other problems of prior art solder compositions are overcomeby the lead free solders of the present invention. The soldercompositions disclosed herein are low solidus temperature,multi-component solder alloys. The principal metals in the alloy are Biand Sn.

In a preferred embodiment of the invention, the low solidus temperature,multi-component solder alloy has major portions of Bi, and Sn, and aneffective amount of a physical and mechanical property enhancing thirdcomponent. The physical and mechanical property enhancing thirdcomponent may be In, Cu, Ag, or combinations thereof.

In an alternative embodiment of the invention the alloy contains atleast about 50 weight percent Bi, up to about 50 weight percent Sn, andan effective amount of a physical and mechanical property enhancingthird component. The third component may be In, Cu, Ag, or a combinationthereof, as Cu and Ag.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a family of lead-free,Sn--Bi solders. These solders are characterized by a low solidustemperature. The principal metals in the alloy are Bi and Sn.

In a preferred embodiment of the invention, the low solidus temperature,multi-component solder alloy has major portions of Bi, and Sn, and aneffective amount of a physical and mechanical property enhancing thirdcomponent. The physical and mechanical property enhancing thirdcomponent may be In, Cu, Ag, or combinations thereof.

In a particularly preferred exemplification of the invention, thesolidus lowering third component is a mixture of Ag and Cu. OneBi--Sn--Cu--Ag alloy composition contains about 46 weight percent Bi,about 48 weight percent Sn, (i.e., at least about 50 weight percent Sn,and less then 50 weight % Bi, basis total Bi and Sn), about 4 weightpercent Cu and about 2 weight percent Ag.

In an alternative embodiment of the invention the alloy contains atleast about 50 weight percent Bi, up to about 50 weight percent Sn, andan effective amount of a physical and mechanical property enhancingthird component. The third component may be In, Cu, Ag, or a combinationthereof, as Cu and Ag.

In one exemplification of this embodiment the solidus lowering thirdcomponent is Cu. One preferred alloy is 48-56 weight percent Bi, 42-48weight percent Sn and 2-4 weight percent Cu.

In an alternative exemplification of this embodiment, the soliduslowering third component is In. A preferred Bi--Sn alloy is onecontaining about 56 weight percent Bi, about 42 weight percent Sn, andabout 2 weight percent In.

These compositions are summarized in the Table below:

    ______________________________________                                        Component        Range (weight %)                                             ______________________________________                                        Sn               42-50 weight %                                               Bi               46-56 weight %                                               ______________________________________                                                         Range (weight %)                                             Optional Component                                                                             when present                                                 ______________________________________                                        Cu               2-4 weight %                                                 In               1-2 weight %                                                 Ag               1-2 weight %                                                 ______________________________________                                    

According to one preferred embodiment of the invention there is provideda method of electrically connecting an integrated circuit chip to acircuitized substrate. This interconnection method includes the step ofdepositing a solder alloy comprising major portions of Bi, and Sn, andan effective amount of a physical and mechanical property enhancingthird component. The physical and mechanical property enhancing thirdcomponent may be In, Cu, Ag, or combinations thereof. This alloy isdeposited onto electrical contacts of the integrated circuit chip. Thesolder alloy may be applied by wave solder deposition,electrodeposition, or as a solder paste.

The electrical leads of the circuitized substrate are then brought intocontact with the solder alloy on the electrical contacts of theintegrated circuit chip. Where the chip is to be mounted in a "flipchip" conformation, the current leads of the circuitized substrate arepads on the substrate, and the solder alloy deposits are brought intocontact with the pads. Alternatively, where the integrated circuit chipis mounted right side up, the current leads are wire leads, and tabinner lead connections, and they are brought into contact with thesolder alloy contacts on the top surface of the integrated circuit chip.

While the substrate current leads and the solder deposits are maintainedin contact the solder alloy is heated to cause the solder alloy to wetand bond to the electrical leads of the circuitized substrate. Heatingmay be by vapor phase reflow, infrared reflow, laser reflow, or thelike.

The resulting microelectronic circuit package of the invention is anintegrated circuit chip module with a circuitized chip carrier, i.e., asubstrate, a semiconductor integrated circuit chip, and an alloy formedof major portions of Bi, and Sn, and an effective amount of a physicaland mechanical property enhancing third component, which may be In, Cu,Ag, or combinations thereof, as an electrical solder bondinterconnection between the circuitized chip carrier and thesemiconductor integrated circuit chip.

EXAMPLE

The invention may be understood by reference to the following example.

A series of 48 weight percent Sn-52 percent Bi solder alloys wereprepared and tested for various solder properties.

In one test the solder alloys were prepared and tested for liquid-soliddigestion properties. In a test of Cu digestion the Cu digested 0.35mils into the liquid solder in 120 seconds. In another test ofliquid-solid digestion, the IMC thickness measured after 100 hours at120 degrees Celsius was 110.

In another test the alloys were tested at 80 degrees Celsius under aload of 200 psi to determine their compression creep properties. Thecompression strength of a 0.006 inch diameter solder ball after 40 hoursin second stage creep was 0.47.

The tensile strength was measured to be 99 (100 lbs/sq. inch), the yieldstrength was measured to be 83 (100 lbs/square inch), and the percentelongation was measured to be 43 percent, all at 0.05 inch per minute.The tensile strength was measured to be 67 (100 lbs/sq. inch), the yieldstrength was measured to be 51 (100 lbs/square inch), and the percentelongation was measured to be 54 percent, all at 0.002 inch per minute.This indicates good high strength properties relative to eutectic Pb/Sn.

The alloys were also tested for cycle fatigue at 4.3 percent deflectionat 23 degrees Celsius. Failure occurred after 3 cycles (five cycles perminute) and 8 cycles (1 cycle per minute).

The melting point of the alloy was measured to be 138 degrees Celsius.

While the invention has been described with respect to certain preferredembodiments and exemplifications, it is not intended to limit the scopeof the invention thereby, but solely by the claims appended hereto.

We claim:
 1. A low solidus temperature, multicomponent solder alloyconsisting essentially of 56 weight percent Bi, 42 weight percent Sn,and 2 weight percent In.
 2. A low solidus temperature, multicomponentsolder alloy consisting essentially of 46 weight percent Bi, 48 weightpercent Sn, 4 weight percent Cu and 2 weight percent Ag.